Apparatus and method for controlling silicon nitride etching tank

ABSTRACT

A method and system for controlling a silicon nitride etching bath provides the etching bath including phosphoric acid heated to an elevated temperature. The concentration of silicon in the phosphoric acid is controlled to maintain a desired level associated with a desired silicon nitride/silicon oxide etch selectivity. Silicon concentration is measured while the silicon remains in soluble form and prior to silica precipitation. Responsive to the measuring, fresh heated phosphoric acid is added to the etching bath when necessary to maintain the desired concentration and silicon nitride:silicon oxide etch selectivity and prevent silica precipitation. The addition of fresh heated phosphoric acid enables the etching bath to remain at a steady state temperature. Atomic absorption spectroscopy may be used to monitor the silicon concentration which may be obtained by diluting a sample of phosphoric acid with cold deionized water and measuring before silica precipitation occurs.

FIELD OF THE INVENTION

The present invention relates, most generally, to semiconductor devicemanufacturing. More particularly, the present invention relates to amethod and system for maintaining a hot phosphoric acid bath used foretching silicon nitride and other semiconductor materials.

BACKGROUND

Silicon nitride is a dielectric material that is very frequently used inmany applications in the manufacture of semiconductor devices. A film ofsilicon nitride is typically formed over a semiconductor substrate uponwhich semiconductor devices are being fabricated. The film is thenpatterned using an etching process that includes phosphoric acid. Thewet chemical etching of silicon nitride has traditionally been doneusing hot phosphoric acid (H₃PO₄) at temperatures of around 160° C. Thebasic chemical reactions that model the etching of silicon nitride withphosphoric acid are:

The silicon nitride etch process is heavily influenced by processparameters including H₃PO₄ and silicon concentration, as evidenced bysilica, SiO₂ precipitation, temperature of the etch bath and the bathlife of the hot phosphoric bath. The oxide (silicon dioxide, SiO₂) etchrate is also affected by process conditions, in particular the siliconconcentration in the etching bath. The oxide etch rate becomesdramatically lower as the Si concentration in the bath increases. The Siconcentration therefore directly affects the silicon nitride:siliconoxide etch selectivity. It is therefore desirable to maintain the bathconditions such as the H₃PO₄ and silicon concentration and the bathtemperature, at constant levels so as to produce constant etch rates andetch selectivities, and process repeatability. As bath life increases,however, these parameters may undesirably vary. It can be seen that itis critical to maintain and control the silicon, Si, concentration inorder to maintain constant silicon nitride and silicon oxide etch ratesand silicon nitride/oxide etch selectivity. Literature indicates thatthe oxide precipitate, i.e., the dehydration of Si₃O₂(OH)₈ to form SiO₂and water, occurs after reaching saturation solubility at about 120 ppmat 165° C. Different temperatures have other saturation solubilitylevels. The generation of oxide precipitates results in a particlesource which is the major yield killer in semiconductor processing.

One known procedure for maintaining a wet silicon nitride etch processis to allow the silica to precipitate and to remove the precipitate bydecreasing the temperature. One silica extraction system is designed toremove the generated silica using lower temperature H₃PO₄ with a high Siconcentration. Shortcomings of this procedure include the difficulty inmaintaining a high extraction efficiency for precipitated silica, whichmay melt into the H₃PO₄ solution again if not removed quickly and ifallowed a long reaction time.

It would therefore be desirable to maintain the etching bath to providerelatively constant etching characteristics without suffering from theaforementioned shortcomings.

SUMMARY OF THE INVENTION

To address these and other needs and in view of its purposes, theinvention provides a method for controlling an etching bath. The methodincludes providing an etching bath containing phosphoric acid heated toan elevated bath temperature, measuring silicon concentration of siliconin the phosphoric acid while the silicon remains in soluble form andcontrolling the silicon concentration by adding fresh heated phosphoricacid to the etching bath when necessary to maintain a desired siliconconcentration, responsive to the measured silicon concentration. Thesilicon is monitored before it precipitates as silica. By maintaining adesired silicon concentration, the silicon nitride:silicon oxide etchselectivity is also maintained within a desired range

According to another aspect, a method for controlling a silicon nitrideetching bath is provided. The method includes providing an etching bathcontaining phosphoric acid heated to an elevated bath temperaturegreater than 120° C., measuring silicon concentration of silicon in thephosphoric acid while the silicon remains in soluble form, and,responsive to the measuring, controlling the silicon concentration tomaintain a silicon nitride:silicon oxide etch selectivity of the etchingbath within a desired range by adding fresh heated phosphoric acid tothe etching bath when necessary while maintaining the elevated bathtemperature so that it does not deviate by greater than two percent.

According to another aspect, an apparatus for controlling an etchingbath includes an etching bath containing phosphoric acid maintained atan elevated bath temperature. The apparatus further includes a measuringsystem that measures silicon concentration of silicon in soluble form inthe phosphoric acid. The apparatus also includes a heater member thatheats the fresh phosphoric acid, means for adding the fresh heatedphosphoric acid to the etching bath and a controller that controlssilicon concentration by causing the heated fresh phosphoric acid to beadded to the etching bath if necessary to maintain a siliconconcentration and therefore the silicon nitride:silicon oxide etchselectivity, within desired ranges.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is best understood from the following detaileddescription when read in conjunction with the accompanying drawing. Itis emphasized that, according to common practice, the various featuresof the drawing are not necessarily to scale. On the contrary, thedimensions of the various features are arbitrarily expanded or reducedfor clarity. Like numerals denote like features throughout thespecification and drawing.

FIG. 1 is a schematic view illustrating the system of the invention;

FIG. 2 is a graph illustrating control of silicon concentration in theetching bath; and

FIG. 3 is a diagram showing the control system of the invention.

DETAILED DESCRIPTION

The invention provides an apparatus and method for controlling siliconnitride etch rates, silicon oxide etch rates and silicon nitride:siliconoxide etch selectivity in a phosphoric acid etching bath by measuringand controlling the silicon concentration in the etching bath. Theapparatus and method measure silicon concentration before silicaprecipitates from the bath. When silicon nitride is etched in a hotphosphoric acid bath, silicon from the silicon nitride is liberated andcomplexes with oxygen and hydroxyl groups and when the concentrationbecomes too high as additional silicon nitride is etched, a silicondioxide (silica, SiO₂) precipitate is formed according to the chemicalreaction shown above. Responsive to the measured silicon concentration,the method and apparatus compare the measured silicon concentration to adesired silicon concentration and determine if phosphoric acid spikingis needed. Spiking is the addition of fresh, pure phosphoric acid to thebath. The addition of the fresh phosphoric acid restores the siliconconcentration in the bath and therefore the etch rates and etchselectivities to desired levels. The invention provides for spiking withheated phosphoric acid in order to maintain a desired, steady-statetemperature and therefore desired etching characteristics of the etchingbath. The spiking method of the invention also presents silicaprecipitation which is a particle source that degrades device yield.

FIG. 1 is a schematic of an exemplary system 100 of the invention.System 100 includes etching bath 2 which contains hot phosphoric acid.In one exemplary embodiment, the hot phosphoric acid, H₃PO₄, may bemaintained at about 150° C. In other exemplary embodiments, the hotphosphoric acid may be maintained at a temperature within the range ofabout 70° C.-160° C. In still another exemplary embodiment, atemperature of 100° C. or greater may be maintained for the H₃PO₄ bath.Various conventional heating members may be used to heat the H₃PO₄ inetching bath 2. In addition to inner tank of etching bath 2, outer tank4 may optionally be present. The apparatus may include recirculationloop 6 into which H₃PO₄ is introduced at outlet port 20. Recirculationloop 6 may include an optional filter 10, heater 12, pump 16 and valve18, although other arrangements may be used in other exemplaryembodiments.According to an exemplary embodiment, the temperature of theH₃PO₄ in etching bath 2 is maintained by heating the H₃PO₄ inrecirculation loop 6 using heater 12, then dispensing the heated, hotH₃PO₄ at inlet port 8. In another exemplary Embodiment (not shown),outer tank 4 may not be needed and alternative means may be used tomaintain the H₃PO₄ in etching bath 2, at desired temperature levels.

Fresh H₃PO₄ is provided at inlet 24 of feed loop 26. Pre-heat tank 28heats the fresh H₃PO₄ such that the H₃PO₄ delivered at inlet 30 is at anelevated temperature, within a range of about 70° C. to about 175° C.,for example. Other temperatures may be used in other exemplaryembodiments. Feed loop 26 also includes pump controller 32 and flowmeter 34. Controller 32, flow meter 34 and pre-heat tank 28 may be incommunication with Si concentration monitoring system 40. Siconcentration monitoring system 40 may draw H₃PO₄ from outer tank 4 byway of port 42 and in another exemplary embodiment, Si concentrationmonitoring system 40 may draw H₃PO₄ from recirculation loop 6 at port44.

Si concentration monitoring system 40 dynamically monitors the siliconconcentration in etch bath 2. The Si concentration is monitored bytaking a sample of H₃PO₄ from either outer tank 4 or recirculation loop6 in the illustrated embodiment but Si concentration monitoring system40 may be in direct contact with the H₃PO₄ in etching bath 2 in otherexemplary embodiments. Semiconductor or other substrates (not shown)with silicon nitride films thereon may be etched in the hot H₃PO₄ inetching bath 2. Various numbers of wafers may be etched at variousfrequencies after the H₃PO₄ is initially dispensed into etching bath 2.The silicon nitride is etched by H₃PO₄ according to:

Conditions of etching bath 2 are maintained so as to prevent the oxideprecipitate, i.e. SiO₂ or silica, from forming. In one exemplaryembodiment, a concentration of no greater than about 60-110 ppm isallowed and in another exemplary a Si concentration no greater thanabout 60 ppm is maintained. The saturated silicon solubility level isnot allowed to be reached thus preventing particles of silicaprecipitates from contaminating etching bath 2 and any product beingetched in etching bath 2. In one exemplary embodiment in which thetemperature of etch bath 2 is about 165° C., Si concentration isprevented from exceeding 120 ppm. In other exemplary embodiments,different ranges of allowable silicon concentration may be maintained.In one exemplary embodiment, the silicon concentration may be maintainedwithin a range of about 110 to 120 ppm to reach the maximum siliconnitride/silicon oxide etch selectivity that may be required for aparticular process.

Si concentration monitoring system 40 measures the Si concentration inthe H₃PO₄. In one exemplary embodiment, atomic absorption spectroscopyis used, but other techniques may be used in other exemplaryembodiments. In one particular embodiment, the hot H₃PO₄ delivered to Siconcentration monitoring system 40 may be diluted with comparativelycold deionized water to form a diluted sample and the diluted samplemeasured for silicon concentration essentially immediately after thedilution, i.e., before silica precipitation occurs. The deionized watermay be at room temperature, i.e. unheated, in one exemplary embodiment.Other techniques may be used in other exemplary embodiments.

Responsive to the measured silicon concentration, Si concentrationmonitoring system 40 communicates with controller 32 to spike the heatedH₃PO₄ into outer tank 4 via port 30 when necessary to adjust siliconconcentration to remain at a desired level, i.e., to suppress siliconconcentration by adding fresh H₃PO₄ which contains no silicon. Thesilicon concentration is related to the silicon nitride etch, thesilicon oxide etch rate and the silicon nitride:silicon oxide etchselectivity of the bath. Si concentration monitoring system 40 maysample and measure H₃PO₄ on a periodic, regular or essentially constantbasis and therefore fresh H₃PO₄ may be added at port 30, responsive tothe measuring at the same frequency. The volume of fresh heated H₃PO₄added to the bath will be determined by the measured siliconconcentration. Different volumes may be added, according to calculationsperformed as will be illustrated below.

Referring to FIG. 2, which is a graph of chemical run number versussilicon concentration, a chemical run may represent an event of etchinga silicon nitride wafer or lot of silicon nitride wafers in etching bath2. Each chemical run may generate one or several data points accordingto one embodiment in which the phosphoric acid is sampled and measuredeach time a wafer or batch of wafers is etched. As SiN is etched,silicon concentration in the bath generally increases. According toconventional tank designs as illustrated by curve 60, the siliconconcentration increases to level 62 at which SiO₂ precipitation occurs.The hot H₃PO₄ bath according to the invention, represented by curve 64,does not approach SiO₂ precipitation levels because fresh H₃PO₄ is addedresponsive to the measured Si concentration when necessary to preventthe Si concentration levels from exceeding level 62. Curve 64 and theetching bath of the invention is maintained within process constrainedarea 66 and SiO₂ precipitation is circumvented, and particlecontamination is prevented.

FIG. 3 is a schematic diagram illustrating the automatic control systemof the invention. Tool 100 provides information 70 by way of H₃PO₄, toSi concentration monitoring system 40 which measures Si concentration inthe H₃PO₄. In one exemplary embodiment, measuring may be done after eachrun, i.e., after a silicon nitride etching operation has been carriedout. Based on the measurement and information 72 provided to controller74, it is determined whether fresh H₃PO₄ needs to be added and, if so,the quantity. In one exemplary embodiment, the amount of fresh H₃PO₄ tobe added may be determined according to the following:C _(t) =C _(n)(50−X)/50

-   -   C_(t):Si concentration target    -   C_(n):Si concentration at run=n    -   Dilution efficiency=50−X/50    -   Tank volume=50 liters, X=spiking volume

In this manner, spiking volume X, if necessary, may be calculated inliters and information regarding the amount to be added provided to tool100 as indicated by arrow 76. Other units may be used in other exemplaryembodiments. In one exemplary embodiment, referring again to FIG. 1, theinformation represented by arrow 76 may be fed to controller 32. C_(t),the Si concentration target, is associated with a siliconnitride:silicon oxide etch selectivity and will be determined by thedesired or target etch rates and selectivity, which may vary. Both theSi concentration and silicon nitride and oxide etch rates and thereforethe nitride:oxide etch selectivity may be controlled to maintain varioustolerances. In one exemplary embodiment, the silicon nitride etch ratemay be maintained so as not to fluctuate by more than 2% of a desired,i.e. target, etch rate, according to the method and system of theinvention. In one exemplary embodiment, the silicon oxide etch rate maybe maintained so as not to fluctuate by more than 2% of a desired, i.e.target, etch rate, according to the method and system of the invention.In one exemplary embodiment, the silicon nitride:silicon oxide etchselectivity may be maintained so as not to fluctuate by more than about5%, preferably about 2%, of a desired etch selectivity according to themethod and system of the invention.

Referring again to FIG. 1, the addition of heated, fresh H₃PO₄ at port30 enables etching bath 2 to maintain an essentially steady statetemperature which may be a temperature within the range of about 125° C.to about 175° C. (preferably within the range of about 130° C. to about170° C.) and maintained at about +/−5° C., preferably +/−2° C. In oneexemplary embodiment, the steady state temperature may not fluctuatemore than 5° C., preferably 2° C., upon addition of heated fresh H₃PO₄.In another exemplary embodiment, the bath temperature may deviate fromthe steady state temperature for a time period less than one minute uponaddition of heated fresh H₃PO₄. Other techniques for controlling the Siconcentration and bath temperature may be used and depending on thelevel of control sought and the control limits of bath temperature andsilicon concentration in place, will determine how frequently and forwhich concentration and temperature variation, fresh, heated H₃PO₄ mustbe added.

The preceding merely illustrates the principles of the invention. Itwill thus be appreciated that those skilled in the art will be able todevise various arrangements which, although not explicitly described orshown herein, embody the principles of the invention and are includedwithin its spirit and scope. For example, the adjustable fresh hotchemical spiking may be determined by theoretical accumulated Siconcentration calculation and not only using the actual Si concentrationmonitor.

Furthermore, all examples and conditional language recited herein areprincipally intended expressly to be only for pedagogical purposes andto aid in understanding the principles of the invention and the conceptscontributed by the inventors to furthering the art, and are to beconstrued as being without limitation to such specifically recitedexamples and conditions. Moreover, all statements herein recitingprinciples, aspects, and embodiments of the invention, as well asspecific examples thereof, are intended to encompass both structural andfunctional equivalents thereof. Additionally, it is intended that suchequivalents include both currently known equivalents and equivalentsdeveloped in the future, i.e., any elements developed that perform thesame function, regardless of structure.

This description of the exemplary embodiments is intended to be read inconnection with the figures of the accompanying drawing, which are to beconsidered part of the entire written description. In the description,relative terms such as “lower,” “upper,” “horizontal,” “vertical,”“above,” “below,” “up,” “down,” “top” and “bottom” as well asderivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,”etc.) should be construed to refer to the orientation as then describedor as shown in the drawing under discussion. These relative terms arefor convenience of description and do not require that the apparatus beconstructed or operated in a particular orientation. Terms concerningattachments, coupling and the like, such as “connected” and“interconnected,” refer to a relationship wherein structures are securedor attached to one another either directly or indirectly throughintervening structures, as well as both movable or rigid attachments orrelationships, unless expressly described otherwise.

Although the invention has been described in terms of exemplaryembodiments, it is not limited thereto. Rather, the appended claimsshould be construed broadly, to include other variants and embodimentsof the invention, which may be made by those skilled in the art withoutdeparting from the scope and range of equivalents of the invention.

What is claimed is:
 1. A method for controlling an etching bathcomprising: providing a batch etching bath configured to process a batchof wafers therein, said batch etching bath containing phosphoric acidheated to an elevated bath temperature, said batch etching bathincluding an inner tank for processing said batch of wafers therein, andan outer tank; measuring silicon concentration of silicon in saidphosphoric acid contained in said batch etching bath while said siliconis in soluble form; and responsive to said measuring, controlling saidsilicon concentration by adding fresh heated phosphoric acid to saidphosphoric acid in said outer tank of said batch etching bath whennecessary to maintain said silicon concentration within a desired range,said fresh heated phosphoric acid heated by a dedicated phosphoric acidheater and wherein said measuring includes removing a sample of saidphosphoric acid and diluting said sample with deionized water to producea diluted sample and measuring said diluted sample before silicaprecipitation in said diluted sample, said deionized water having atemperature no greater than room temperature.
 2. The method as in claim1, wherein said controlling prevents silica precipitation.
 3. The methodas in claim 1, wherein said controlling further maintains a siliconnitride:silicon oxide etch selectivity of said batch etching bath so asnot deviate by more than 2% from a target selectivity.
 4. The method asin claim 1, wherein said elevated bath temperature is maintained withina range of about 130-170° C. and does not deviate by more than 2% duringand after said adding fresh heated phosphoric acid to said outer tank ofsaid batch etching bath.
 5. The method as in claim 1, wherein said freshheated phosphoric acid includes a temperature within a range of about70° C.-160° C.
 6. The method as in claim 1, wherein each of saidmeasuring and said controlling, are carried out periodically.
 7. Themethod as in claim 1, wherein said measuring silicon concentrationcomprises measuring said silicon concentration using atomic absorptionspectroscopy.
 8. The method as in claim 1, further comprising etchingsilicon nitride in said batch etching bath thereby producing saidsilicon in said batch etching bath via chemical reaction.
 9. The methodas in claim 1, wherein said controlling further maintains a siliconoxide etch rate of said batch etching bath within a desired range. 10.The method as in claim 1, wherein said measuring silicon concentrationincludes circulating said phosphoric acid from and to said batch etchingbath, heating said circulating phosphoric acid and measuring saidcirculating phosphoric acid.
 11. The method as in claim 1, wherein saidcontrolling said silicon concentration by adding fresh heated phosphoricacid maintains a steady state temperature condition in said batchetching bath.
 12. The method as in claim 1, wherein said controllingsaid silicon concentration to maintain a silicon nitride:silicon oxideetch selectivity of said batch etching bath within a desired range byadding fresh heated phosphoric acid, maintains a steady statetemperature condition in said batch etching bath.
 13. The method as inclaim 1, wherein said elevated bath temperature reaches a steady statetemperature that is between about 140° C.-160° C. and remains within arange of +/−2° C. and said elevated bath temperature does not deviatefrom said steady state temperature for more than 1 minute during andafter said adding fresh heated phosphoric acid to said outer tank ofsaid batch etching bath.
 14. The method as in claim 13, furthercomprising maintaining said silicon concentration at about 60 ppm orless.
 15. A method for controlling an etching bath comprising: providinga batch etching bath configured to process a batch of wafers therein,said batch etching bath containing phosphoric acid heated to an elevatedbath temperature, said batch etching bath including an inner tank forprocessing said batch of wafers therein, and an outer tank; measuringsilicon concentration of silicon in said phosphoric acid contained insaid batch etching bath while said silicon is in soluble form; andresponsive to said measuring, controlling said silicon concentration byadding fresh heated phosphoric acid to said phosphoric acid in saidouter tank of said batch etching bath when necessary to maintain saidsilicon concentration within a desired range, said fresh heatedphosphoric acid heated by a dedicated phosphoric acid heater, whereinsaid measuring silicon concentration includes circulating saidphosphoric acid from and to said batch etching bath, heating saidcirculating phosphoric acid and measuring said circulating phosphoricacid by removing a sample of said circulating phosphoric acid anddiluting said sample with deionized water to produce a diluted sampleand measuring said diluted sample before silica precipitation in saiddiluted sample, said deionized water having a temperature no greaterthan room temperature.
 16. A method for controlling a silicon nitrideetching bath comprising: providing a batch etching bath including aninner tank configured to process a batch of wafers therein and an outertank, said batch etching bath containing phosphoric acid heated to anelevated bath temperature greater than about 120° C.; measuring siliconconcentration of silicon in said phosphoric acid contained in said batchetching bath while said silicon is in soluble form; and responsive tosaid measuring, controlling said silicon concentration to maintain asilicon nitride:silicon oxide etch selectivity of said batch etchingbath within a desired range by adding fresh heated phosphoric acid tosaid phosphoric acid in said outer tank of said batch etching bath whennecessary to maintain said silicon nitride etch rate within said desiredrange, while maintaining said elevated bath temperature so that saidelevated bath temperature does not deviate by greater than 5% from asteady state value of said elevated bath temperature, said fresh heatedphosphoric acid heated by a dedicated phosphoric acid heater anddelivered to said batch etching bath unmixed with other solutions,wherein said measuring includes removing a sample of said phosphoricacid and diluting said sample with deionized water to produce a dilutedsample and measuring said diluted sample before silica precipitation insaid diluted sample, said deionized water having a temperature nogreater than room temperature.